As a result of the steep price rise of fossil energy in recent years, there is a demand for systems that can generate electric power directly from natural energy. Thus, solar cells that use single crystal, polycrystalline or amorphous Si; solar cells of compound systems such as GaAs or CIGS (a semiconductor material comprising copper (Cu), indium (In), gallium (Ga) and selenium (Se)); dye-sensitized photoelectric conversion elements (Graetzel cells), and the like have been proposed and put to practical use.
However, the cost of generating electric power using these solar cells is still higher than the price of electricity generated and transmitted using fossil fuels, and this high cost has been a hindrance to wide distribution of the solar cells. Furthermore, since heavy glass must be used in the substrate, reinforcing works are required at the time of installation, and these also constitute a factor of increasing the power generation cost.
Under such circumstances, a bulk heterojunction type photoelectric conversion element having a photoelectric conversion layer as a mixture of an electron donor layer (p-type semiconductor layer) and an electron receptor layer (n-type semiconductor layer) between a transparent electrode and a counter electrode has been proposed as a solar cell which is capable of achieving a power generation cost that is lower than the power generation cost made by fossil fuels, and it has been reported that the photoelectric conversion element exhibits an efficiency of higher than 5% (see, for example, Non-Patent Literature 1).
In regard to solar cells using these bulk heterojunction type photoelectric conversion elements, since members other than an anode and a cathode can be formed by coating, rapid production at low cost is enabled, and there is a possibility that the problems of power generation cost described above may be solved. Furthermore, unlike the Si-based solar cells, semiconductor-based solar cells, dye-sensitized solar cells and the like, since there is no need for a production process at a temperature higher than 160° C., it is expected that forming a solar cell on an inexpensive and lightweight plastic substrate is also feasible.
However, in addition to the increase of efficiency directed to practicalization, there is also a demand for an enhancement of durability. In regard to such a problem, since it is known that durability is enhanced in a solar cell of a type in which a metal that does not easily cause deterioration of electrodes and the like and has a high work function is used as a counter electrode, and the solar light incident side is used as a cathode (so-called inversion layer solar cell) (see, for example, Patent Literature 1), there is a demand for a material that can exhibit high photoelectric conversion efficiency in an inversion layer configuration.
However, an inversion layer solar cell has a configuration that is disadvantageous from the viewpoint of light utilization, in connection with the fact that a conductive polymer layer having poor light transmissibility is present between a metal electrode and an electricity generating layer. Therefore, it is required from a simulation that the optimal film thickness be larger compared to an ordered layer solar cell (see, for example, Non-Patent Literature 2). Accordingly, there is a demand for a material which generates electricity even in the form of a thick film (approximately 150 nm or more). Many materials exhibit satisfactory efficiency when fabricated into electricity generating layers of thin films (100 nm or less); however, when fabricated into electricity generating layers of thick films (100 nm or more), many materials have decreased fill factors (FF), so that there is a problem that there are very few materials that can achieve high efficiency.
Recently, in Non-Patent Literature 3, it has been reported that power generation can be achieved with an efficiency of 6% by a polymer having a naphthobisthiadiazole group. This report concerns an ordered layer configuration, but it is expected that when an organic thin film solar cell having an inversion layer configuration is produced, a solar cell having a good balance between high photoelectric conversion efficiency and durability would be obtained.